The present invention relates in general to radiation dosimeters, and in particular to a new and useful MOS dosimeter which can measure a dosage of radiation using non-destructive changes or variations which can be measured and with the dosimeter being resettable for taking a new measurement.
MOS dosimeters of known kind may be used particularly as sensors for monitoring areas exposed to radiation, such as area of nuclear power plants, for personal protection in such environments, or space vehicles, and as a dosimeter for positional resolution by determining the energy dose by location and time.
The principle of an MOS dosimeter is known, for example, from the article "A New Direct Reading Solid-State Dosimeter" by Ian Thomson (Canadian Astronautics Limited, 28, Annual Conference of the Health Physics Society, June 1983, Baltimore, Md.). According to this article, electron-hole pairs are generated in the oxide of MOS structures under ionizing irradiation. If a voltage is applied during the irradiation to the gate contact of the MOS structure, a part of the electron-hole pairs in the electric field of the oxide is spatially separated so fast that they escape a recombination. Under a positive gate voltage, the electrons leave the oxide through the gate contact, while the holes migrate to the Si/SiO.sub.2 interface. In the zone of this interface, a part of the holes, which are movable at room temperature, may be captured in neutral hole traps, so that a positive oxide charge Q.sub.fr is formed. For the threshold voltage variation .DELTA.U.sub.th of an MOS transistor occurring in the lower and medium dose region (D&lt;50 Krad(SiO.sub.2)), the following equation applies while neglecting the effect of interface states: ##EQU1## Wherein .epsilon..sub.o is the dielectric constant of a vacuum, .epsilon..sub.SiO.sbsb.2 is the dielectric constant of the oxide, and x is the location of the charge centroid K is the ionization coefficient indicating the generated hole charge per unit volume and per rad (SiO.sub.2). The charge yield factor f.sub.H indicates the proportion of electron-hole pairs which do not recombine after their generation. It holds that: EQU 0.ltoreq.f.sub.H .ltoreq.1 (1a)
D stands for the absorbed dose, and d.sub.ox for the thickness of the oxide layer. A indicates the proportion of holes which get captured in the neutral traps wherein 0.ltoreq.A.ltoreq.1.
It follows from equation (1) that: EQU .DELTA.U.sub.th .about.D (2)
This proportionality between the absorbed dose D and the threshold voltage shift .DELTA.U.sub.th is varied as long as the number of holes captured in neutral traps is small relative to the number of hole traps capable of trapping. The relation (2) makes it possible to use MOS varactors or MOS transistors as dosimeters. Such MOS dosimeters may be considered prior art.
The use of such a simple MOS dosimeter has the following disadvantages: The measuring process involves damaging of the MOS structure. This damage is caused by capturing holes in traps in the Si/SiO.sub.2 interface zone, and although it changes the threshold voltage .DELTA.U.sub.th as desired, other electrical properties of the MOS structure are thereby impaired. For example, fast and slow interface states are produced. The fast interface states reduce the slope of the MOS transistor and unfavorably affect the dynamic properties. The slow interface states make the C(U)-curve hysteretic, with the result of instability in the transistor characteristic. The sensitivity (.DELTA.U.sub.th /D) of the dosimeter is governed and determined by the hole trapping factor A and depends on numerous imponderable factors in the MOS fabrication process. A sensitive MOS dosimeter must be capable of capturing as many holes as possible. This means that an oxide which is sensitive to radiation must be employed as the insulator layer, collecting a high positive charge already under a small dose of irradiation. Such a sensitive oxide, however, is irreversibly damaged by the radiation, so that the characteristics become unstable (slow interface states). The prior art MOS dosimeter cannot be electrically reset in a simple way. Resetting is possible only through high temperature treatment, for example, which is not desirable however.
The mentioned drawbacks of prior art MOS dosimeters are also the reason why it is not possible to integrate on a chip MOS dosimeter transistors with signal processing electronic elements, without risking damage to these electronic elements. In addition, only negative displacements of the C(U) characteristic can be evaluated, since only positive oxide charges can be collected.